Field of the Invention
The invention relates to a process for operating a PEM fuel cell installation.
It is known that, during the electrolysis of water, water molecules are decomposed by electric current into hydrogen (H.sub.2) and oxygen (O.sub.2). In a fuel cell, that process takes place in reverse. Electric current is produced with high efficiency through an electrochemical combination of hydrogen (H.sub.2) and oxygen (O.sub.2) to form water. If pure hydrogen (H.sub.2) is used as combustion gas, it takes place without the emission of pollutants and carbon dioxide (CO.sub.2). Even with a technical combustion gas, for example natural gas or coal gas, and with air (which may additionally be enriched with oxygen (O.sub.2)) instead of pure oxygen (O.sub.2), a fuel cell produces considerably less pollutants and less carbon dioxide (CO.sub.2) than other forms of energy production which operate by using fossil energy sources. The technical implementation of the fuel cell principle has given rise to a wide variety of solutions, and more precisely with different electrolytes and with operating temperatures of between 80.degree. C. and 1000.degree. C.
The fuel cells are classified as low, medium and high temperature fuel cells according to their operating temperature, and they in turn differ for a variety of technical embodiments.
Besides the aforementioned fundamental advantages, a fuel cell with a plastic solid electrolyte (polymer electrolyte membrane or PEM) offers further positive properties such as low operating temperature (&lt;80.degree. C.), favorable response to overloading, little voltage degradation and long life span, favorable response to loading cycles and temperature cycles, and the absence of a corrosive liquid electrolyte. It is furthermore suitable for operation with ambient air instead of oxygen (O.sub.2). Together, those properties make an air-operated PEM fuel cell a virtually ideal producer of energy, for example for electrically powering a motor vehicle without producing exhaust gases.
A PEM fuel cell block (the fuel cell block is also referred to as a "stack" in the specialist literature) is generally composed of a large number of PEM fuel cells which have a planar structure and are stacked together. Since the PEM fuel cell block is not operable on its own, the PEM fuel cell block, an operating part and associated module electronics are generally combined to form a PEM fuel cell module. The devices for supplying working media, for example hydrogen (H.sub.2) and oxygen (O.sub.2) or air, for discharging the water which is produced, for dissipating heat losses, for wetting the working media and for separating gas impurities, are combined in the operating part.
If the anode and the cathode of the PEM fuel cell are supplied with their working media, then a cell voltage is created from the sum of the anode and cathode potentials, which has a specific characteristic depending on the load current. In order to ensure an orderly build-up of all of the cell voltages in the PEM fuel cell block when switching on, the supplying of the anodes and cathodes with their working media must be established through a defined switch-on phase, so as to make it possible to obtain a delay-free changeover from the switch-on phase to a phase for producing electrical energy, in other words, a load phase. When switching off, the supplying of the PEM fuel cell block with the working media is suspended, and the residual capacity in the gas spaces of the anodes and cathodes is taken down by loading, so long as the voltage of the PEM fuel cell block remains greater than 0 V.
In one process for switching off a PEM fuel cell module, which is known from the prior art, a hydrogen inlet valve is closed in a first step and an oxygen inlet valve is closed in a second step, according to the hydrogen partial pressure at an anode part. That process leads to an increase in the internal resistance of the PEM fuel cells and poisoning of the electrolyte membrane, which is equivalent to a power loss and causes premature failure of the PEM fuel cell module.